The next few months were spent in relative quiet, the brigade either training or helping in the construction of defensive positions in expectation of a repeat of the trench warfare that had characterised so much of World War I. By 9 May 1940, the day before the German Armyattacked in the West, Stopford's brigade, along with the rest of the 5th Division, was held in GHQ Reserve, the War Office's view being that it should return to the United Kingdom as a reserve. However, by 16 May the division (excluding the 15th Brigade, which had been removed for participation in operations in Norway) was on the River Senne, where it first encountered the Germans, but was soon ordered to disengage and withdraw to the River Escaut. On 19 May the division was ordered to Arras, where a gap was emerging.[12] Major General Franklyn, GOC of the 5th Division, was ordered to take command of Major General Giffard Martel's 50th Division and the 1st Army Tank Brigade, in addition to his own division, which was to be known as "Frankforce". On 21 May "Frankforce" was ordered by GeneralLord Gort, Commander-in-Chief (C-in-C) of the BEF, to attack across the Gernan line of advance. Stopford's 17th Brigade was held in reserve on Vimy Ridge for the operation, and, on 23 May, after Stopford himself noticed German infantry and tanks advancing on 17th Brigades' position. Although French support was promised it never materialised and the brigade, after heavy fighting, was ordered to retreat, withdrawing from their positions on the night of 23 May and the early hours of 24 May.[12]

The 5th Division was then moved to the Ypres−Comines Canal, where another gap had been created on the BEF's left flank, due to the wholesale surrender of the Belgian Army.[13] Stopford's brigade came under a succession of very heavy attacks from 26 to 28 May, suffering very heavy losses as a result, but managing to retain its position. By the time the 17th Brigade fell back towards Dunkirk, from where it was evacuated to England on the night of 31 May/1 June, the brigade was reduced from a strength of over 2,500 officers and men, at the beginning of the campaign, to less than that of a single battalion, and Brigadier Dempsey's 13th Brigade was in a similarly depleted state.[13]

He then became Commandant of the Staff College, Camberley in October 1941, and was promoted to temporary major general in January 1942.[16] Ironically Stopford's predecessor, Major General Robert Collins, had been one of his instructors there when attending as a student in the 1920s.[1]

Mountbatten was not beaten and, upon returning to India, ordered Stopford to continue to train XXXIII Corps in amphibious operations, which it did so for the next months. In March 1944, however, the Japanese 15th Army, under Lieutenant GeneralRenya Mutaguchi, launched an offensive at the centre of the Allied front at Imphal. Lieutenant General William Slim, GOC of the Fourteenth Army (which Stopford's XXXIII Corps was serving under), and Lieutenant General Geoffrey Scoones, GOC British IV Corps, had both predicted a move like this by the Japanese, and Scoones, whose IV Corps was holding the sector, withdrew his corps into a more defensible sector.[19] The Fourteenth Army's GOC had failed to estimate the arrival of the Japanese 31st Division under Lieutenant General Kōtoku Satō, which headed for Kohima, 80 miles north of Imphal. If Satõ's 31st Division were able to take the small town of Kohima, they would be almost unopposed and be able to march into Assam, thereby cutting land communications to Ledo, at the Indian end of the Ledo Road, then in the process of being built to China.[19]

The British first received reports that the Japanese were aiming for Kohima from the local Naga people, and from V Force patrols, in the third week of March. The 1st Battalion, Assam Regiment moved west to Jessami to intercept them. On 28 March fighting began and continued for another two days, gaining valuable time. The battalion, only very recently raised, fighting against a numerically superior force, was forced to withdraw to Kohima. At the same time Colonel Hugh Richards had arrived to take command of the garrison at Kohima, which was severely outnumbered.[19] Lieutenant General Slim, GOC Fourteenth Army, made a decision for the 161st Indian Brigade, detached from Major General Harold Briggs's 5th Indian Division, to be flown into Dimapur, and to move into Kohima, arriving there on 29 March, after receiving reports on the Japanese strength. Slim also placed Major General R. P. L. Ranking, GOC 202nd Lines of Communication Area (202 LoC), in temporary command of the area.[19]

Realising that Lieutenant General Scoones, GOC British IV Corps, would be unable to control the Kohima battle, Slim asked his superior, GeneralSir George Giffard, commanding the 11th Army Group, for Stopford and his HQ XXXIII Corps to be flown out from India. Stopford, establishing his HQ on 3 April at Jorhat, took over from Major General Ranking, and began to assess the situation. He outlined the priorities as being, in the following order, Dimapur, the Ledo Road and Kohima, and ordered the 161st Indian Brigade to defend the Nichugard Pass, thereby safeguarding Dimapur but leaving Kohima uncovered.[19] However, the priorities were changed, following a consultation with the Fourteenth Army GOC, and Kohima was now made the first priority, and the 161st Indian Brigade was ordered to return. Only a battalion, the 4th Queen's Own Royal West Kent Regiment (4th Royal West Kents), and a single company of the 4th Battalion, 7th Rajput Regiment managed to reinforce the Kohima garrison, which consisted of a significant number of non-combat troops, before the town was surrounded. The remainder of the 161st Indian Brigade was cut off at Jotsoma, a few miles back from the road to Dimapur.[19]

Lieutenant General Montagu Stopford, GOC XXXIII Indian Corps (right), confers with Major General John Grover, GOC 2nd Division (left) and Brigadier Joseph Salomons, commanding the 9th Indian Brigade (centre), after the opening of the Imphal-Kohima road, June 1944.

The initial defence of Kohima, therefore, was conducted by a far smaller force than was necessary. However, fighting in very grim conditions reminiscent of World War I, the force managed to hold on during a siege that lasted over two weeks, the British and Indian troops being boxed in on Garrison Hill. The distance between them and the Japanese was the length of the local District Commissioner's tennis court. Three Indian mountain batteries at Jotsoma were initially the only outside support.[19] However, Major General Grover's British 2nd Division broke the road block between Jotsoma and Dimapur, thus enabling the 161st Indian Brigade to relieve the defenders of Kohima on 18 April.

Stopford's objective was now to drive the Japanese away from Kohima, the British 2nd Division being the main initial tool for the job, although significant reinforcements were on the way. These consisted of the 23rd Brigade, which had been intended to join the Chindits, and the 21st Indian Division, temporarily created under the command of Major General Cameron Nicholson, who Stopford knew as a fellow instructor at the Staff College before the war, to take command of other units who had been brought up from India.[19] The 6th Brigade of the British 2nd Division relieved the 161st Indian Brigade and continued to hold Garrison Hill against a succession of Japanese assaults. The division's 4th Brigade undertook a right hook flanking movement to come in from the south, against the Aradura Spur, while the 5th Brigade began a left hook from the north. Both brigade assaults did not meet with the expected success. The 33rd Indian Brigade, commanded by Brigadier Frederick Loftus-Tottenham, part of Major General Frank Messervy's 7th Indian Division, arrived from the Arakan and, assisted by the 6th Brigade, cleared the enemy from Kohima Ridge.[20] However, the fighting was relentless as the Japanese remained in two strong defensive preparations, on the Aradura Ridge and around Naga Village. The British 2nd Division, which by now had suffered very heavy casualties, pushed them off Aradura Ridge in early June, the 7th Indian Division pushing them out of Naga Village around the same time. The Japanese retreated to the Chindwin River and, on 22 June, Stopford's XXXIII Corps finally made contact with Lieutenant General Scoones's British IV Corps.[20]

Lieutenant General Sir Montagu Stopford, GOC-in-C of the British 12th Army, inspects a guard of honour mounted by men of the 1st Battalion, Royal Berkshire Regiment, during the formal ceremony in Rangoon where General Heitarō Kimura and his staff handed over their swords to staff officers of the 12th Army.

After the war Stopford served as commander of Burma Command (renamed from Twelfth Army) from 1945 to 1946, as C-in-C Allied Land Forces in the Dutch East Indies in 1946 and as C-in-C SEAC from 1946 to 1947 before becoming General Officer Commanding-in-Chief (GOC-in-C) of Northern Command in England from 1947 to 1949.[5] He retired from the British Army in 1949,[5] with the rank of full general. He was also appointed Colonel-in-Chief of the Rifle Brigade.[20]